Our bodies contain far more microbial genes than human genes. And a new study suggests that just as human DNA varies from person to person, so too does the massive collection of microbial DNA in the intestine. The research is the first to catalog the genetic variation of microbes that live in the gut, where they extract nutrients from food, synthesize vitamins, protect against infections, and produce compounds that naturally reduce inflammation. The widespread genetic diversity uncovered by the scientists can help them understand how our microbial genes work together with our human genes to keep us healthy or, in some cases, to cause disease.